1. Field of the Invention
This invention relates to an air-fuel ratio-control system for an internal combustion engine having air-fuel ratio sensors provided at respective locations upstream and downstream of a catalytic converter arranged in the exhaust system of the engine, which control system controls the air-fuel ratio of an air-fuel mixture supplied to the engine, based on outputs from the air-fuel ratio sensors.
2. Prior Art
There is conventionally known a method of controlling the air-fuel ratio of an air-fuel mixture supplied to the engine, which method comprises providing air-fuel ratio sensors at respective locations upstream and downstream of a three-way catalyst arranged in the exhaust system of the engine, and carrying out feedback control of the air-fuel ratio of the mixture supplied to the engine (hereinafter referred to as "the supplied air-fuel ratio"), based on outputs from the air-fuel ratio sensors.
Further, an air-fuel ratio control system has been proposed, for example, by Japanese Provisional Patent Publication (Kokai) No. 63-147941, which carries out feedback control of the supplied air-fuel ratio, based on an output from an air-fuel ratio sensor arranged downstream of a catalytic converter arranged in the exhaust system of an internal combustion engine, and which can deal with deterioration of the catalytic converter by detecting a deterioration degree of the catalyst of the catalytic converter, and changing a skip amount (proportional term) and an integral term applied in the feedback control in response to the detected deterioration degree.
However, the above conventional control system is merely based on the fact that deterioration of the catalyst leads to a degradation in the O2 storage capacity of the catalytic converter, resulting in a shortened repetition period of change of the feedback control amount. Accordingly, the above control system which changes the skip amount and the integrated term merely in order to compensate for the shortened repetition period suffers from the following disadvantage:
That is, the deteriorated catalyst cannot completely purify unburnt gas components in the exhaust gases, so that some of the unburnt gas components (CO and HC) pass through the catalyst to the downstream side thereof and then to the downstream air-fuel ratio sensor. CO and HC as the unburnt gas components undergo a reaction with oxygen molecules from the atmosphere side which have passed through a zirconia layer of the air-fuel ratio sensor, at a platinum electrode of the catalyst. Due to the reaction, the downstream air-fuel ratio sensor outputs a signal indicative of a rich supplied air-fuel ratio even when residual oxygen molecules remain in the exhaust gases (the supplied air-fuel ratio is lean). As a result, the air-fuel ratio feedback-controlled based on the output from the downstream air-fuel ratio sensor deviates to a leaner value than a desired value thereof, thereby unfavorably increasing NOx emission.